1. Field of the Invention
The present invention relates generally to the fields of blood vessels and of coagulation. More particularly, it provides a variety of growth factor-based and immunological reagents, including bispecific antibodies, for use in achieving specific coagulation.
2. Description of the Related Art
Advances in the chemotherapy of neoplastic disease have been realized during the last 30 years. This includes some progress in the development of new chemotherapeutic agents and, more particularly, the development of regimens for concurrent administration of drugs. A significant understanding of the neoplastic processes at the cellular and tissue level, and the mechanism of action of basic antineoplastic agents, has also allowed advances in the chemotherapy of a number of neoplastic diseases, including choriocarcinoma, Wilm's tumor, acute leukemia, rhabdomyosarcoma, retinoblastoma, Hodgkin's disease and Burkitt's lymphoma. Despite the advances that have been made in a few tumors, though, many of the most prevalent forms of human cancer still resist effective chemotherapeutic intervention.
A significant underlying problem that must be addressed in any treatment regimen is the concept of "total cell kill." This concept holds that in order to have an effective treatment regimen, whether it be a surgical or chemotherapeutic approach or both, there must be a total cell kill of all so-called "clonogenic" malignant cells, that is, cells that have the ability to grow uncontrolled and replace any tumor mass that might be removed. Due to the ultimate need to develop therapeutic agents and regimens that will achieve a total cell kill, certain types of tumors have been more amenable than others to therapy. For example, the soft tissue tumors (e.g., lymphomas), and tumors of the blood and blood-forming organs (e.g., leukemias) have generally been more responsive to chemotherapeutic therapy than have solid tumors such as carcinomas.
One reason for the susceptibility of soft and blood-based tumors to chemotherapy is the greater physical accessibility of lymphoma and leukemic cells to chemotherapeutic intervention. Simply put, it is much more difficult for most chemotherapeutic agents to reach all of the cells of a solid tumor mass than it is the soft tumors and blood-based tumors, and therefore much more difficult to achieve a total cell kill. Increasing the dose of chemotherapeutic agents most often results in toxic side effects, which generally limits the effectiveness of conventional antitumor agents.
The strategy to develop successful antitumor agents involves the design of agents that will selectively kill tumor cells, while exerting relatively little, if any, untoward effects against normal tissues. This goal has been elusive to achieve, though, in that there are few qualitative differences between neoplastic and normal tissues. Because of this, much research over the years has focused on identifying tumor-specific "marker antigens" that can serve as immunological targets both for chemotherapy and diagnosis. Many tumor-specific, or quasi-tumor-specific ("tumor-associated"), markers have been identified as tumor cell antigens that can be recognized by specific antibodies. Unfortunately, it is generally the case that tumor specific antibodies will not in and of themselves exert sufficient antitumor effects to make them useful in cancer therapy.
More recently, immunotoxins have been employed in an attempt to selectively target cancer cells. Immunotoxins are conjugates of a specific targeting agent, typically a tumor-directed antibody or fragment, with a cytotoxic agent, such as a toxin moiety. The targeting agent is designed to direct the toxin to cells carrying the targeted antigen and to kill such cells. "Second generation" immunotoxins have now been developed, for example, those that employ deglycosylated ricin A chain to prevent entrapment of the immunotoxin by the liver and reduce hepatotoxicity (Blakey et al., 1987a;b), and those with new crosslinkers to endow the immunotoxins with higher in vivo stability (Thorpe et al., 1988).
Immunotoxins have proven effective at treating lymphomas and leukemias in mice (Thorpe et al., 1988; Ghetie et al., 1991; Griffin et al., 1988a;b) and in man (Vitetta et al., 1991). However, lymphoid neoplasias are particularly amenable to immunotoxin therapy because the tumor cells are relatively accessible to blood-borne immunotoxins. Also, it is possible to target normal lymphoid antigens because the normal lymphocytes, which are killed along with the malignant cells during therapy, are rapidly regenerated from progenitors lacking the target antigens.
In contrast with their efficacy in lymphomas, immunotoxins have proved relatively ineffective in the treatment of solid tumors (Weiner et al., 1989; Byers et al., 1989). The principal reason for this is that solid tumors are generally impermeable to antibody-sized molecules: specific uptake values of less than 0.001% of the injected dose/g of tumor are not uncommon in human studies (Sands et al., 1988; Epenetos et al., 1986). Another significant problem is that antigen-deficient mutants can escape being killed by the immunotoxin and regrow (Thorpe et al., 1988).
Furthermore, antibodies that enter the tumor mass do not distribute evenly for several reasons. Firstly, the dense packing of tumor cells and fibrous tumor stromas present a formidable physical barrier to macromolecular transport and, combined with the absence of lymphatic drainage, create an elevated interstitial pressure in the tumor core which reduces extravasation and fluid convection (Baxter et al., 1991; Jain, 1990). Secondly, the distribution of blood vessels in most tumors is disorganized and heterogeneous, so some tumor cells are separated from extravasating antibody by large diffusion distances (Jain, 1990). Thirdly, all of the antibody entering the tumor may become adsorbed in perivascular regions by the first tumor cells encountered, leaving none to reach tumor cells at more distant sites (Baxter et al., 1991; Kennel et al., 1991).
Thus, it is quite clear that a significant need exists for the development of novel strategies for the treatment of solid tumors. One approach involves the targeting of agents to the vasculature of the tumor, rather than to tumor cells. Solid tumor growth is highly dependent on the vascularization of the tumor and the growth of tumor cells can only be maintained if the supply of oxygen, nutrients and other growth factors and the efflux of metabolic products are satisfactory. Indeed, it has been observed that many existing therapies may already have, as part of their action, a vascular-mediated mechanism of action (Denekamp, 1990).
The present inventors propose that targeting the vasculature will likely deprive the tumor of life sustaining events and result in reduced tumor growth rate or tumor cell death. This approach is contemplated to offer several advantages over direct targeting of tumor cells. Firstly, the target cells are directly accessible to intravenously administered therapeutic agents, permitting rapid localization of a high percentage of the injected dose (Kennel et al., 1991). Secondly, since each capillary provides oxygen and nutrients for thousands of cells in its surrounding `cord` of tumor, even limited damage to the tumor vasculature could produce an avalanche of tumor cell death (Denekamp, 1990; Denekamp, 1984). Finally, the outgrowth of mutant endothelial cells, lacking a target antigen, is unlikely because they are normal cells.
At the present time, it is generally accepted that for tumor vascular targeting to succeed, antibodies are required that recognize tumor endothelial cells but not those in normal tissues. Although several antibodies have been raised (Duijvestijn et al., 1987; Hagemeier et al., 1986; Bruland et al., 1986; Murray et al., 1989; Schlingemann et al., 1985), none have shown a high degree of specificity. Also, there do not appear to be reports of any particular agents, other than the aforementioned toxins, that show promise as the second agent in a vascular targeted antibody conjugate. Thus, unfortunately, while vascular targeting presents certain theoretical advantages, effective strategies incorporating these advantages have yet to be developed.